On cross-shore beach profile morphodynamics

Resumen

The nearshore zone is plenty of 3D morphodynamic patterns resulting from the interaction of waves, currents and sediments. The dynamics of formation and evolution of these patterns is at present a controversial point in coastal research. However, frequently nearshore zone shows a strongly persistent uniformity. In this situation, morphodynamic changes are dominated just by crossshore processes. Remarkably, although this situation displays much less morphological complexity, cross-shore beach profile morphodynamics is still a research challenge, as sediment transport, in this case, is the result of a very subtle balance between onshore and offshore directed forces that still remain unsolved. The aim of this thesis is to get more insight on the physical processes involving cross-shore beach profile evolution and how relevant are them in the nearshore zone morphodynamics. To this end, a 1D non-linear morphodynamical model for the evolution of the profile is developed to analyze some relevant aspects of cross-shore beach profile morphodynamics. The AMORFO70 model couples hydrodynamics, sediment transport and bottom changes to account for the morphodynamics feedback. The model considers the depth-integrated and wave-averaged momentum and mass conservation equations coupled with wave- and roller-energy conservation, Snell's law and the dispersion relation under the assumption of alongshore uniformity. It is well-known that the intra-wave processes, particularly the near-bottom orbital velocity and acceleration, can lead to net onshore sediment transport. The model accounts for the most novel parameterization of the near-bed intra-wave velocity, to analyze the effects of the temporal distribution of the intra-wave near-bottom velocities and accelerations on cross-shore morphodynamics. It is found that accounting for both velocity and acceleration skewness in the sediment transport is essential to properly simulate onshore sandbar migration and the entire profile evolution. Results have shown a strong spatial dependence of sediment transport along the profile, in such a way that in the shoaling zone transport is mostly driven by bed-shear stress (velocity skewness) and the breaking and inner-zone transport is dominated by pressure gradients (acceleration skewness). The accurate description of sediment transport is a key issue in morphodynamic modeling. The model has been complemented with several transport parameterizations to analyze the differences between morphodynamic predictions related to different sediment transport formulas for different sequences. Results evidenced several differences between the predicted transport rates and also between the predicted incipient bottom changes of the different sediment transport formulas. It is found that the cross-shore morphodynamic predictions depend strongly on the sediment transport formula that is used and not all of them capture the expected trends. Particularly, formulas that account directly for the effects of wave velocity and acceleration skewness lead to the best predictions, especially for accretionary sequences. A common procedure on cross-shore beach profile morphodynamic modeling is to neglect the alongshore variability. This assumption has been analyzed for the prediction of the mean profile evolution on the short-, the mid- and the long-term. It has been proven that the model is able to reproduce short- and mid-term evolution of the mean profile with substantial accuracy. Thus, considering the mean profile as the average of the evolution of individual profiles along the shoreline leads to the best results, as 'a way to account for the alongshore variability'. In the long-term it is found that, although predictions may agree with measurements, they do not capture the real morphodynamics. This stresses the relevance of analyzing the behavior of the simulated morphodynamics during long-term evolution to avoid mistakes in the interpretation of the model capabilities.